This invention is related generally to steam turbines and generators, and more particularly to a method and apparatus for aligning the rotor of such steam turbines or generators, wherein the rotor is supported at its ends by a pair of journal bearings.
Journal bearings for steam turbine and generator rotors must be supported in their housings by an arrangement that is sufficiently stiff such that it does not influence the vibrational characteristics of the rotor train, and yet provides enough mobility to allow the bearing to be aligned to the proper horizontal and vertical location to support the rotor. If the bearing support arrangement is not stiff or has a variable stiffness dependent on the installation procedure, the rotor train vibrational characteristics could be affected undesirably resulting in lateral critical speeds becoming resonant with rotational speed. Because the rotor train must be accurately aligned to reduce misalignment bending shaft stresses and bearing pressures, the bearing support arrangement must also provide a means of vertical as well as horizontal motion capability to accomplish adequate alignment.
Large steam turbines, such as the BB73 model manufactured by Westinghouse Electrical Corporation, typically include a rotor and a pair of journal bearings supporting the rotor at its ends, each of the journal bearings having an upper half and a lower half adapted to be joined together forming a bearing shell which is mounted within a spherical bore of a bearing pedestal or support. Since the rotors of such turbines must of necessity operate in an aligned condition, various means have been provided in the past to support the journal bearings within their respective bearing supports. One prior art approach which provides such support is commonly referred to in the art as a three-key bearing support system.
In the widely-used three-key bearing support system, the upper half of each journal bearing includes a first keyway, formed longitudinally therein with respect to the rotor to contain a bearing support pad or key having an outer male spherical surface adapted to conform to the spherical bore in the bearing support. Second and third keyways are formed longitudinally in the lower half of the journal bearing to respectively contain second and third bearing support pads or keys. Such second and third keyways are often spaced radially 90.degree. from one another substantially equidistant from a plane which bisects the lower half radially. Like the first bearing support pad or key, the second and third bearing support pads or keys include an outer male spherical surface which is adapted to substantially conform to the spherical bore in the bearing support.
Each of the three bearing support pads or keys, prior to assembly, have in the past been rigidly attached within their respective keyways. Typically, such rigid attachment is provided by threadedly coupling the bearing support pads or keys to their respective keyways. In order to align the rotor, however, alignment shims or liners are first inserted as necessary between the bearing support pad or key requiring adjustment in its keyway. Thereafter, the outer male spherical surfaces of each bearing support pad or key is handworked by filing or scraping the surface to provide a predetermined percentage of contact between the surface and the spherical bore of the bearing pedestal or support. Since the bearing support pad or key which is installed in the upper half of each journal bearing is substantially free of the weight of the rotor, the fit of the bearing support pad or key in the upper half of the journal bearing is not critical and, more often than not, a 10% contact between the bearing support pad or key installed in the upper half of the journal bearing and the spherical bore is suitable. On the other hand, at least 75% contact is often required for the bearing support pads or keys installed in the lower half of the journal bearing. It can be readily appreciated, therefore, that changes of alignment shims or liners will necessitate reworking of the outer male spherical surfaces of the bearing support pads or keys installed in the lower half of the journal bearings in order to maintain the at least 75% contact.
Such changes of alignment shims or liners may be required to align the rotor when changing conditions such as the settling of the turbine's or generator's foundation, whereon adjacent bearings, or distortion in the turbine or generator as a whole, cause the rotor to become misaligned. Alignment of the rotor must also be checked, with changes to alignment shims or liners made as necessary, when the rotor has been removed from the turbine or generator for maintenance. Because the bearing support pads or keys in prior art approaches have been rigidly attached to the journal bearing, changes increasing or decreasing the thickness of the alignment shims or liners create radii which are greater or less than the radii before the change. That is, the true radius from the center of a rotor to the outer male spherical surface of a bearing support pad or key is increased or decreased when alignment shims or liners are added or removed. These added or removed alignment shims or liners cause a change of position of the bearing support pad or key within its keyway such that changes in the percentage of contact between the outer male spherical surface of the bearing support pad or key and the spherical bore result. Therefore, changes of alignment shims or liners often require repeated handworking of the outer male spherical surfaces of the bearing support pads or keys, especially those installed in the lower half of the journal bearing to maintain at least 75% contact. Such repeated handworking in large steam turbines or generators may often consume hours or even days of effort.
In the recent past, a newer method and apparatus for supporting tilt pad journal bearings have been devised. This approach replaces the spherical pads and mating circumferential groove (i.e., the spherical bore) with a pair of triangular keys and mating V-grooves. The rationale behind this concept was that once the triangular keys were fitted into their grooves, no future refitting of contact surfaces would be necessary for alignment or realignment. Any alignment move would be made with rectangular shims having complex tapers. Nevertheless, during installation and alignment of the triangular keys, considerable difficulty was experienced in the past in obtaining proper contact between the triangular key and its mating V-groove due to the difficulty of field machining the V-grooves into the bearing support with true perpendicular flat surfaces, considerable time was spent in hand-scraping the fitted surfaces to achieve proper contact. Furthermore, during subsequent alignment moves, the triangular key shifted in its groove because of the use of improperly tapered shims, resulting in unacceptable contact gaps between the triangular keys and V-grooves. More often than not, the rotor had to be removed, the bearing lifted, and the keys refit into the bearing support. An excessive amount of time was, therefore, spent on the fitting of the triangular keys into their V-grooves. As a result, alignment of the rotor became more difficult due to the bad contact between the keys and grooves further resulting in an inability to obtain repeatable alignment moves.